1. Field of the Invention
The present invention relates to a driving circuit for a liquid crystal display (LCD) backlight which enables synchronization of integrated circuits conducting pulse width modulation (PWM) control. More particularly, the present invention relates to a driving circuit for an LCD backlight which is applied to respective PWM controlling integrated circuits to synchronize the phases of driving signals from the PWM controlling integrated circuits, thereby synchronizing the PWM controlling integrated circuits together and preventing interference signals and noises that may otherwise occur due to asynchronization.
2. Description of the Related Art
In general, with a market for liquid crystal display (LCD) TVs and monitors expanding, LCD backlights are also growing in their size. Also, in order to enhance driving capacity of lamps such as a Cold Cathode Fluorescent Lamp (CCFL), a need has arisen to adopt at least two integrated circuits (ICs) in parallel. Here, in case of driving the integrated circuits, driving signals thereof should be synchronized in phases.
FIG. 1 is a configuration view illustrating a conventional LCD backlight driver.
Referring to FIG. 1, the conventional LCD backlight driver includes a time constant circuit 1, a first PWM controlling integrated circuit IC1 and a second PWM controlling integrated circuit IC2. The time constant circuit 1 provides time constant τ to determine a frequency of a driving signal. The first PWM controlling integrated circuit IC1 generates a first driving signal Sd1 in accordance with the time constant of the time constant circuit 1. The second PWM control integrated circuit IC2 generates a second driving signal Sd2 in accordance with the time constant of the time constant circuit 1.
The time constant circuit 1 includes a time constant capacitor CT for determining the time constant, and optionally a time constant resistor. The time constant capacitor CT and the time constant resistor can be variously connected to the time constant circuit 1.
Moreover, the first and second PWM controllers IC1 and IC2 each include a driving circuit for an LCD backlight for generating the driving signal. The driving circuit for the LCD backlight will be explained with reference to FIG. 2.
FIG. 2 is a configuration view illustrating a driving circuit for an LCD backlight in PWM controlling integrated circuits.
Referring to FIG. 2, the conventional driving circuit for the LCD backlight in the PWM controlling integrated circuits includes a fundamental wave generator 10, a PWM comparator 20, and a driving signal generator 30. The fundamental wave generator 10 generates a triangle wave and a square wave in accordance with time constant. The PWM comparator 20 compares a difference signal Vref between a feedback voltage Vfb and a preset reference voltage Vref to generate a PWM signal Spwm. The driving signal generator 30 generates a driving signal sd in response to the square wave from the fundamental wave generator 10 and the PWM signal Spwm from the PWM comparator 20.
FIG. 3 is a timing chart illustrating major signals of the driving circuit for the LCD backlight of FIG. 1. In FIG. 3, Sero denotes a difference signal between a feedback voltage Vfb and a reference voltage Vref, Sosc denotes a triangle wave signal generated from the fundamental wave generator 10, Sd denotes a driving signal, which includes a high signal HO and a low signal LO.
Referring to FIG. 3, as described above, the conventional driving circuit for the LCD backlight generates the driving signal including the high and low signals HO and LO in accordance with the difference signal Sero between the feedback voltage Vfb and the preset reference voltage Vref, and the triangle square wave signal Sosc.
In greater detail, referring to FIGS. 2 and 3, to generate the driving signal from the conventional driving circuit for the LCD backlight, the square wave from the fundamental wave generator 10 is compared with the PWM signal Spwm from the PWM comparator 20. Here, the high signal HO (or low signal LO) is outputted in the first period and the low signal LO (or high signal) is outputted in the second period. That is, the high and low signals HO and LO are outputted randomly without any priority given thereto.
As described above, in the conventional driving for the LCD backlight in which the at least two PWM controlling integrated circuits are driven in parallel, the driving signals are out of synchronization even with use of the same triangle wave signals since there is no priority between the high and low signals.
Accordingly, the driving signals are outputted randomly from the PWM controlling integrated circuits, failing to synchronize the output signals from the PWM controlling integrated circuits together. This causes interference of light and occurrence of noises in lamps such as the CCFL. This results in defects in the LCD backlight such as flickering.